Measurement report: Sources, sinks and lifetime of NO_X in a sub-urban temperate forest at night

Abstract. Through observations of NO, NO₂, NO_Y and O₃ in the Rambouillet forest near Paris, France, (as part of the ACROSS campaign, 2022) we have gained insight into nighttime processes controlling NO_X in an anthropogenically impacted forest environment. O₃ mixing ratios displayed a strong diel profile at the site, which was driven by a variable but generally rapid deposition to soil and foliar surfaces. The O₃ diel profile was strongly influenced by relative humidity, which impacted the surface resistance to uptake, and temperature inversion, which influenced the rate of entrainment of O₃ from above the canopy. Only when the O₃ mixing ratio was sufficiently low (and thus the NO lifetime sufficiently long), were sustained NO peaks observed above the instrumental detection limit, enabling derivation of average NO emission rates from the soil of ~1.4 ppbv h^‑1. Observations of the lack of increase in NO₂ at night, despite a significant production rate from the reaction of NO with O₃, enabled an effective lifetime of NO₂ of ⁓0.5–3 h to be derived. As the loss of NO₂ was not compensated by the formation of gas- or particle-phase reactive nitrogen species it was presumably driven by deposition to soil and foliar surfaces, or any products formed were themselves short-lived with respect to deposition. By comparison, the daytime lifetime of NO₂ with respect to loss by reaction with OH is about 1 day. We conclude that the nighttime deposition of NO₂ is a major sink of boundary layer NO_X in this temperate forest environment.